Fuel Fittings - Thread Sealant

Mostly flare fittings. Makes sense. Odd fuel system. Worthy of replacement! Where are the drops for sumping the low points? I skimmed so maybe I overlooked that? Did you flow test it before and after?
 
Stewartb said:
I wonder how many guys who replied actually have built or installed an aircraft fuel system? I know a couple of the pro mechanics have and some of their replies have me scratching my head.
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Just because they’re a “pro” doesn’t necessarily make them right or good at fuel system repairs. Their talents may lie elsewhere. ;)

Some of the comments leave me wondering a bit as well.
 
Some of the responses above allude to an important, yet basic fact about NPT threads. The threads are tapered, and if properly cut, the joint becomes an interference seal when the proper torque is applied. No clearance remains between the peaks and roots of the threads because of the taper.

Pipe thread sealants perform an important function. They lubricate the joint, which allows the joint to be tightened further than dry threads would.

This is the primary function of pipe sealants. They don't act as a gap displacement filler or adhesive. They simply provide lubrication, and as the joint is tightened, the thread taper reduces the spiral gap between the peaks and valleys of the threads to an interference fit.

Of course, thread sealant compounds do provide a mechanical barrier to the passage of fluids and gases, albeit on a microscopic level. However, in most cases, this is a superfluous action, incidental to the lubricating function. The thread taper is the primary means of making a leak tight joint.
 
3393RP said:
Some of the responses above allude to an important, yet basic fact about NPT threads. The threads are tapered, and if properly cut, the joint becomes an interference seal when the proper torque is applied. No clearance remains between the peaks and roots of the threads because of the taper.

Pipe thread sealants perform an important function. They lubricate the joint, which allows the joint to be tightened further than dry threads would.

This is the primary function of pipe sealants. They don't act as a gap displacement filler or adhesive. They simply provide lubrication, and as the joint is tightened, the thread taper reduces the spiral gap between the peaks and valleys of the threads to an interference fit.

Of course, thread sealant compounds do provide a mechanical barrier to the passage of fluids and gases, albeit on a microscopic level. However, in most cases, this is a superfluous action, incidental to the lubricating function. The thread taper is the primary means of making a leak tight joint.
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All true, but in many cases you don't dare tighten that fitting as much as it would take to seal it without sealant; you'd crack the pipe boss. The bowl drains on the Marvel Schebler/Precision Airmotive carbs are a prime example of that. The 1/8" pipe plug will easily crack that boss.

Or you can get galling of the threads; any time an aluminum fitting goes into an aluminum body, there's going to be trouble without some anti-seize of some sort. The galling can happen on installation, and certainly on removal. Think about the oil cooler line fittings in some oil coolers; some OEMs used aluminum fittings instead of the steel they should have used, and I have had unlubricated/unsealed threads tear out of the cooler when trying to remove the fittings.

And when installing elbows, orientation is important, and most of the time the fitting isn't going to end up pointing where you need it, so you have to back it off or tighten it further, depending on what the assembly can tolerate. Sealant, particularly a locking sealant, is handy there.

I wish pipe threads had gone away a long time ago. Larger airplanes have been using O-ring sealed straight threads for ages (MS series), which eliminate the need for goopy sealants and the residue they leave, and allow orientiation of elbows with no trouble at all. They seal first time, every time, and disassembly is easy. You'll find them on Dukes or Weldon fuel boost pumps, for instance, but otherwise we're still finding pipe threads.

conductors1_4.gif
 
Stewartb said:
Where are the drops for sumping the low points? I skimmed so maybe I overlooked that? Did you flow test it before and after?
Click to expand...
Sumps are in the corners of the tanks and the gascolator on the firewall. There were no drains in the original setup where the fuel lines ran down under the doors and then back up - one of the reasons for rework.
Didn't bother to flow check since all of the fittings have a (significantly) larger ID and the total run was shorter. So, there was no reason to believe that the flow would be reduced.
 
Capt. Geoffrey Thorpe said:
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Nice video. The one thing that caught my eye was the troublelight. I hate them with a passion - burned my hands more than once. They get too hot, are hard to get in the right place and the bulb is exposed, which is a shatter hazard. I prefer the sealed fluorescent sticks - they are light and can be taped into the needed position. Also the LED style.
 
Dan Thomas said:
All true, but in many cases you don't dare tighten that fitting as much as it would take to seal it without sealant; you'd crack the pipe boss. The bowl drains on the Marvel Schebler/Precision Airmotive carbs are a prime example of that. The 1/8" pipe plug will easily crack that boss.

Or you can get galling of the threads; any time an aluminum fitting goes into an aluminum body, there's going to be trouble without some anti-seize of some sort. The galling can happen on installation, and certainly on removal. Think about the oil cooler line fittings in some oil coolers; some OEMs used aluminum fittings instead of the steel they should have used, and I have had unlubricated/unsealed threads tear out of the cooler when trying to remove the fittings.

And when installing elbows, orientation is important, and most of the time the fitting isn't going to end up pointing where you need it, so you have to back it off or tighten it further, depending on what the assembly can tolerate. Sealant, particularly a locking sealant, is handy there.

I wish pipe threads had gone away a long time ago. Larger airplanes have been using O-ring sealed straight threads for ages (MS series), which eliminate the need for goopy sealants and the residue they leave, and allow orientiation of elbows with no trouble at all. They seal first time, every time, and disassembly is easy. You'll find them on Dukes or Weldon fuel boost pumps, for instance, but otherwise we're still finding pipe threads.

conductors1_4.gif
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Your points are well founded. I didn't intend my post to be taken as stating thread sealers and lubricants aren't needed.
 
JAWS said:
Nice video. The one thing that caught my eye was the troublelight. I hate them with a passion - burned my hands more than once. They get too hot, are hard to get in the right place and the bulb is exposed, which is a shatter hazard. I prefer the sealed fluorescent sticks - they are light and can be taped into the needed position. Also the LED style.
Click to expand...

I much prefer, and have used for years, the LED headlights. The light is always where you're looking, no burn or ignition hazards, and the batteries last a long time. My favorite is the Eveready:

images


That's not me. I'm much older and more handsome...:)
 
3393RP said:
Some of the responses above allude to an important, yet basic fact about NPT threads. The threads are tapered, and if properly cut, the joint becomes an interference seal when the proper torque is applied. No clearance remains between the peaks and roots of the threads because of the taper.
Click to expand...

This is not correct. NPT threads have, by design of the threadform tolerancing, a spiral leak path that requires a sealant to be leak-free. The standard for NPT threads is ASME B1.20.1. Quoting directly from the specification:

1.4 Sealing (NPT and NPSC Only)

1.4.1 Mating Threads. Mating threads should
always contact on the thread flanks. The design tolerances
are such that mating crests and roots may clear,
contact, or interfere (see Fig. 1). This joint may not necessarily
seal, unless a sealant is used.

1.4.2 Sealant. Where pressure-tight, leak-free joints
are required, it is intended that threads conforming to
this Standard be made up wrench-tight with a sealant.
To prevent galling during installation, the sealant may
have lubricating properties.

There is a different tapered threadform, NPTF Dryseal, that guarantees a leak-free joint. This threadform is under a different specification, ASME B1.20.3.
 
Dan Thomas said:
I much prefer, and have used for years, the LED headlights. The light is always where you're looking, no burn or ignition hazards, and the batteries last a long time. My favorite is the Eveready:

images


That's not me. I'm much older and more handsome...:)
Click to expand...

I've got about three of those, absolutely the best money I've ever spent on lighting for maintenance.
 
Correct, like I posted a week ago but people can't be bothered with facts lol. NPTF actually has two classes of thread form precision, class 1 still needs sealant, class 2 it truly "dry seal." See: http://greensladeandcompany.com/wp-...hreads/Threads - NPTF Taper Thread Gaging.pdf The problem is there are many tapered thread forms with various classes of fit, however most people just use "NPT" to cover them all when they should be more specific.

slacktide said:
This is not correct. NPT threads have, by design of the threadform tolerancing, a spiral leak path that requires a sealant to be leak-free. The standard for NPT threads is ASME B1.20.1. Quoting directly from the specification:

1.4 Sealing (NPT and NPSC Only)

1.4.1 Mating Threads. Mating threads should
always contact on the thread flanks. The design tolerances
are such that mating crests and roots may clear,
contact, or interfere (see Fig. 1). This joint may not necessarily
seal, unless a sealant is used.

1.4.2 Sealant. Where pressure-tight, leak-free joints
are required, it is intended that threads conforming to
this Standard be made up wrench-tight with a sealant.
To prevent galling during installation, the sealant may
have lubricating properties.

There is a different tapered threadform, NPTF Dryseal, that guarantees a leak-free joint. This threadform is under a different specification, ASME B1.20.3.
Click to expand...
 
Well, once again stuff I've been told is accurate turns into pie on my face. :oops:
 
IMG_1445.JPG "I wonder how many guys who replied actually have built or installed an aircraft fuel system? I know a couple of the pro mechanics have and some of their replies have me scratching my head."

Okay so here is a shot of my 20 year old can!
 
Continental's 31 Jul 2017 M-0 Standard Practice Maintenance Manual Table 3-4 Sealants p11/40

657042 Loctite 565 Adhesive Sealant
Use on all pipe threads to oil coolers and other oil sources All models (use sparingly on male threads only)

Loctite 592 Teflon PS/T Pipe Sealant
Use on all pipe threads except as noted elsewhere
All pressure relief valve housing threads Permold 2 studs engine mount 1-3-5
side bottom All models, where applicable
All threaded fasteners installed in a stud hole through to an oil source
Apply before installing threaded fastener

And below that in Table 3-5 in the Lubricant section:
646943 Loctite 76732 Anti-Seize Lubricant Fuel injector nozzles (at cylinder head)
 
Dan Thomas said:
No difference. Exactly the same MIL spec as Fuel Lube and EZ Turn.
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Not true. The mil-spec stuff is a heavy wax, unlike the greasy stuff you can buy in the tube. Does the same thing, but different.
 
Tom-D said:
Probably just like Bondrite and Alodine 1200, same mil-spec, different stuff. does the same thing.
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It does stiffen with age. I've seen it. The tech sheet pdf I posted gives the specs for the AMS-G-6032: consistency is soft, NLGI number is 3, and so on. It all starts out as the sticky grease. We had an old can of it that was far stiffer, more like hard honey. Probably 30 years old. There are some solvents in it that gradually vaporize.
 
Dan Thomas said:
It does stiffen with age. I've seen it. The tech sheet pdf I posted gives the specs for the AMS-G-6032: consistency is soft, NLGI number is 3, and so on. It all starts out as the sticky grease. We had an old can of it that was far stiffer, more like hard honey. Probably 30 years old. There are some solvents in it that gradually vaporize.
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I am old enough to remember opening new cans of the "Grease Valve Plug" when I was in the NAVY. The stuff in the can was a lot stiffer than any thing we get in a tube now days.
 
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